1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic method such as a copying machine, and particularly to an image forming apparatus of the intermediate transfer method in which an endless intermediate transfer belt is provided and toner images formed on the intermediate transfer belt are transferred onto a recording material to obtain output images.
2. Description of the Related Art
Conventionally, a number of image forming apparatuses of the intermediate transfer method, in which toner images with different colors formed on a photoreceptor are primarily transferred in a sequentially superimposed manner onto the endless intermediate transfer belt, and the toner images formed on the intermediate transfer belt are secondarily transferred onto a recording material, have been proposed.
In such an image forming apparatus, such as in Japanese Patent Laid-Open No. 3-288167, deviation or skew is apt to occur on the belt while the intermediate transfer belt is driven and it is necessary to prevent damage to the belt by biasing, etc. As a unit for controlling the bias of the belt, a technology in which the belt bias condition is detected by a detection unit installed around the belt and a biasing force is given to the belt by force so that the belt makes reciprocal movement reliably within a given range has been proposed.
Japanese Patent Laid-Open Nos. 2003-330300 and 6-118823 also use technology in which the central portion of a support member of a roller holding a belt has a crown shape formed with a large diameter and the belt is made to follow the shape of the roller to generate internal stress difference between the end portion and the central portion of the belt, so that the belt is deviated toward the center.
Nevertheless, if the deviation control of the endless belt as described in the above-mentioned patent documents is applied to the intermediate transfer belt mentioned above, there is a possibility of the following problems occurring. That is, if the intermediate transfer belt is moved by the deviation control in the width direction that is perpendicular to the running direction, the positions of the toner image and the recording material may deviate in a width direction until the toner image formed on the photoreceptor is transferred onto the recording material through the intermediate transfer belt.
As a result, there is a possibility of a problem in which the widths of the left and right margins of the printed material are different. In the case of outputting duplex printed material in which images are formed on both sides of the recording material, there is a possibility of another problem in which a good duplex printed material cannot be output because the image position of the obverse and the image position of the reverse do not match due to the different widths of the left and right margins on the obverse and the reverse.